This invention relates to the art of wireless communications, and more particularly, to wireless communication systems using multiple antennas at the transmitter and multiple antennas at the receiver, so called multiple-input multiple-output (MIMO) systems.
It is well known in the art that multiple-input multiple-output (MIMO) systems can achieve dramatically improved capacity as compared to single antenna, i.e., single antenna to single antenna or multiple antenna to single antenna, systems. However, to achieve this improvement, it is preferable that there be a rich scattering environment, so that the various signals reaching the multiple receive antennas be largely uncorrelated. If the signals have some degree of correlation, and such correlation is ignored, performance degrades and capacity is reduced.
We have invented a way of developing signals in a MIMO system such that even in the face of some correlation the most open-loop capacity that can be achieved using a channel of that level of correlation is obtained. In accordance with the principles of the invention, the signals transmitted from the various antennas are processed so as to improve their ability to convey the maximum amount of information. More specifically, the data to be transmitted is divided into M+1 substreams, where M is the number of transmit antennas. Each transmit antenna is supplied with a combination signal that is made up of a weighted version of a common one of the substreams and a weighted version of a respective one of the substreams that is supplied uniquely for that antenna, so that there are M transmit signals. A receiver having N antennas receives the M transmit signals as combined by the channel and reconstitutes the original data therefrom. This may be achieved using successive decoding techniques. Advantageously, the open-loop capacity, i.e., the rate of information that can be conveyed with an arbitrarily small probability of error when the instantaneous forward channel condition is unknown to the transmitter, is maximized.
In one embodiment of the invention, the weights are determined by the forward channel transmitter using channel statistics of the forward link which are made known to the transmitter of the forward link by being transmitted from time to time from the receiver of the forward link by the transmitter of the reverse link. In another embodiment of the invention, a determination of weight parameter, or the weights themselves, is made by the forward channel receiver using the channel statistics of the forward link and the determined weight parameter, or weights, is made known to the transmitter of the forward link by being transmitted from time to time from the receiver of the forward link by the transmitter of the reverse link.